The present invention relates to a method and an apparatus for detecting coordinates selected using a pressure-sensitive resistive digitizing tablet employed in digitizing and providing for stylus computer input. More particularly, the present invention relates to a method and apparatus for initiating a coordinate detection operation and simplifying the circuitry required therefor.
Pressure-sensitive resistance tablets have been used for providing stylus inputs to computers, for example in the area of computer automated design where blueprints and schematics are entered into a computer database. Such tablets have an X-coordinate resistance board facing a Y-coordinate resistance board separated by small insulating beads interposed between the two boards. The two boards are placed in contact with one another at a point of contact where a stylus, or other object, exerts pressure deflecting one board into contact with the other. The X and Y coordinate resistance boards each have a reference electrode on a first edge and a ground electrode on an opposing second edge. Each board is oriented such that a potential gradient in each is oriented in its respective direction, X and Y respectively. Each of the reference electrodes is selectively connected to V.sub.cc via a switch, and each of the ground electrodes is selectively connected to ground via a switch. Additionally, the ground electrode of the Y-coordinate board is connected to an X-input of a selector switch and the ground electrode of the X-coordinate board is connected to a Y-input of the selector switch. The selector switch selectively applies one of the X and Y inputs to a pull-down resistor, selectively connected to ground by a grounding switch, and an A/D converter input. All the switches are controlled by a CPU.
In order to determine the coordinates of the point of contact the CPU operates the switches so as channel voltages to the A/D converter representative of the X and Y coordinates. To measure the X coordinate, V.sub.cc and ground are applied to the X-coordinate board via the respective switches and the selector switch is set to X-input which connects to the ground electrode of the Y-coordinate board. All other switches remain open, including the grounding switch. In this configuration, the X-coordinate board functions as a resistive divider wherein a point of division is the point of contact. The potential at the point of contact is applied to the A/D converter via the Y-coordinate board and its grounding electrode (which is not connected to ground at this time), and the selector switch. The resistance of the Y-coordinate board is inconsequential because the A/D converter has a high impedance input and the pull-down resistor is disconnected from ground during this operation. Thus, the A/D converter digitizes the potential at the point of contact. The CPU then reads the digitized potential and determines an X-coordinate. Determination of the Y-coordinate is achieved similarly by closing switches to apply V.sub.cc and ground to the Y-coordinate board and opening switches applying the same to the X-coordinate board.
During operation the CPU must determine when inputs are made to the resistive tablet, that is, when a stylus is applied to the tablet, so that it can initiate the above coordinate determination process. In the disclosure of Japanese laid-open patent publication SN 4-80411, this task is accomplished by sensing a voltage across the pull-down resistor when the grounding switch of the pull-down resistor is closed to ground, V.sub.cc is applied to the input electrode of the X-coordinate resistance board, the selector switch is set to the X-input, and the remaining switches are set open. Basically, the pull-down resistor applies a ground potential to the A/D converter when there is no point of contact on the tablet. Without the pull-down resistor, the input to the A/D converter floats because the Y-coordinate board, whose grounding electrode is connected to the input of the A/D converter, is floating since both switches to its input electrode and grounding electrode are open. When a point of contact is made, a potential, other than ground, is applied to the pull-down resistor and the A/D converter input because the input electrode of the X-coordinate board has V.sub.cc applied to it by the closed switch. This potential is sensed by the A/D converter and the CPU initiates the coordinate measurement routine described above.
The above apparatus and method of measurement initiation has several drawbacks. First it requires an additional switch and resistor for grounding the A/D input. Second, the CPU must provide for control of the additional switch. And third, if the grounding switch were to be eliminated and the pull-down resistor permanently grounded, the advantage of the high input impedance of the A/D converter would be negated. This would lead to errors being introduced by stray resistances in connections leading to the A/D input and the lower impedance would introduce an additional voltage drop across the resistive tablet thereby lowering the resolution of the system.
Finally, the prior art digitizing apparatus often detects pressure using one of the X and Y coordinate boards as a resistive divider connected between V.sub.cc and ground with another one of the X and Y coordinate boards connected to the input of the A/D converter. In this configuration, the coordinate board connected as a resistive divider continuously bleeds current from V.sub.cc to ground. In battery powered applications, this current drain can significantly reduce an expected charge life of the batteries. Thus, room for improvement exists in construction and operation methods of resistive tablets.